Optimal asymmetry of transistor-based terahertz detectors

TL;DR

This paper demonstrates that geometric asymmetry in graphene FETs significantly enhances terahertz responsivity, revealing an optimal asymmetry configuration that maximizes detector performance across different detection mechanisms.

Contribution

It introduces the concept of optimal asymmetry in FETs for THz detection and experimentally verifies the maximized responsivity with shifted gate position.

Findings

Responsivity peaks when the gate is shifted toward the source.

Universal maximization condition for large channel resistance.

Differential behavior of thermoelectric and self-mixing voltages in small resistance regime.

Abstract

Detectors of terahertz radiation based on field-effect transistors (FETs) are among most promising candidates for low-noise passive signal rectification both in imaging systems and wireless communications. However, it was not realised so far that geometric asymmetry of common FET with respect to source-drain interchange is a strong objective to photovoltage harvesting. Here, we break the traditional scheme and reveal the optimally-asymmetric FET structure providing the maximization of THz responsivity. We fabricate a series of graphene transistors with variable top gate position with respect to mid-channel, and compare their sub-THz responsivities in a wide range of carrier densities. We show that responsivity is maximized for input gate electrode shifted toward the source contact. Theoretical simulations show that for large channel resistance, exceeding the gate impedance, such recipe for responsivity maximisation is universal, and holds for both resistive self-mixing and photo-thermoelectric detection pathways. In the limiting case of small channel resistance, the thermoelectric and self-mixing voltages react differently upon changing the asymmetry, which may serve to disentangle the origin of nonlinearities in novel materials.